Iron oxides have been used for years in foundry applications to improve core properties and the quality of castings. Iron oxides have proven to be advantageous as an additive to foundry molding aggregates containing silica sand to improve the quality of castings by reducing the formation of thermal expansion defects, such as veining, scabs, buckles, and rat tails as well as gas defects, such as pinholes and metal penetration. There are several iron oxides which are currently used in foundries today. These include red iron oxide, also known as hematite (Fe2O3), black iron oxide, also known as magnetite (Fe3O4) and yellow ochre. Another iron oxide which is presently being used is Sierra Leone concentrate which is a hematite ore black in color. Red iron oxide and black iron oxide are the most popular iron oxides in use.
The currently accepted method of employing the above iron oxides is to add approximately 1-3% by weight to the sand mold aggregates during mixing. The exact mechanism by which iron oxides affect surface finish is not totally understood. However, it is generally believed that the iron oxides increase the hot plasticity of the sand mixture by the formation of a glassy layer between the sand grains which deforms and “gives,” without fracturing at metallurgical temperatures, to prevent fissures from opening up in the sand, which in turn reduces veining.
Various other types of additives have also been employed in an attempt to improve core properties and the quality of sand castings. For example, other anti-veining compounds which have been used in sand aggregate mixtures include starch based products, dextrin, fine ground glass particles, red talc and wood flour, i.e. particles of wood coated with a resin. All of these additives have met with limited success in reducing veining.
Currently, minerals containing lithia are utilized in the glass, glaze, and enamel industries as a fluxing agent. Also, in Nakayama et al, U.S. Pat. No. 5,057,155, a lithium mineral is added to a mold-forming composition to function as an expansive agent during heating and firing of ceramic molds used in the investment casting industry. According to Nakayama et al, the mold-forming composition irreversibly expands during firing of the mold in proportion to the amount of lithium mineral present to provide dimensional accuracy for castings by compensating for solidification shrinkage which occurs during cooling of poured metals such as titanium and the like used, for example, in dental castings. However, Nakayama et al fails to teach using a lithia-containing compound such as α-spodumene as an anti-veining agent in sand-based foundry molding and core mixtures.
U.S. Pat. No. 5,911,269 to Brander et al., which is incorporated herein by reference, teaches a method of making a silica sand-based foundry mold wherein thermal expansion defects, i.e. veining, are reduced by adding a lithia-containing material in a sufficient amount to the silica sand mold to provide about 0.001% to about 2.0% of lithia, wherein the addition of lithia is accomplished by adding lithium bearing minerals. A sand-based aggregate of silica sand, binder, and lithia-containing material is disclosed, where the silica sand comprises from about 80% to about 90% of the aggregate, the binder contains about 0.5-10% of the aggregate, and the lithia-containing material provides from about 0.001% to about 2.0% of lithia. The addition of lithia is accomplished by adding lithium bearing materials such as α-spodumene, amblygonite, montebrasite, petalite, lepidolite, zinnwaldite, eucryptite or lithium carbonate.
A specific formulation of a lithia additive as disclosed in Brander et al. was developed, and is commercially known as “Veinseal 14000.” The formulation for Veinseal 14000 is: 68.00% lithia-based material; 7.00% metallic oxide; 25.00% “filler material.” The filler material is TiO2-containing ilmenite. The Veinseal 14000 product is an effective anti-veining agent that is used at a minimum effective concentration of about 5% based on sand weight (B.O.S.) of the sand cores.
U.S. Pat. No. 6,972,302 to Baker et al. teaches an anti-veining material comprising less than about 4% by weight of a lithia-containing material, and at least about 1% by weight of ferric oxide (Fe2O3), with the anti-veining material preferably comprising 2.5% Li2O, 10-25% of TiO2, 15-25% Al2O3, 10-25% of Fe3O4, and 60-70% of SiO2 mixed with about 1% by weight of Fe2O3, preferably red iron oxide. In Baker et al., thermal expansion of sand cores and unwanted veins in the metal casting formed thereby are substantially eliminated with the use of less than 4% by weight of lithia-containing anti-veining agents, such as the Veinseal 14000 product, combined with the use of an effective amount of ferric oxide (Fe2O3), at least about 1% by weight, thereby reducing the quantity of lithia-containing anti-veining agent needed by adding ferric oxide, resulting in a reduction in cost without a decrease in the quality of the castings.
The effective temperature ranges for the effectiveness of the additives in the prior art are not fully detailed. In the casting process, non-ferrous alloys (aluminum, brass, bronze, etc.) are poured between 1200 and 2200° F., molten iron is poured between 2450 and 2750° F., and steel is poured between 2750 and 3000° F. Brander recites temperatures in the 2600-2800° F. range in experiments in which iron was poured. Neither Brander nor Baker discuss the performance of the lithia-containing materials plus metallic oxides blend when they are exposed to higher temperatures, like those in the steel casting process.
The commercial product, Veinseal 14000, is ineffective in preventing the thermal expansion defects in foundry sand cores that are used to make steel castings. When this formula is implemented to combat thermal expansion in cores used in the steel casing process, the results achieved when used with iron castings are not achieved at the higher steel temperature. Instead, the veining defect is prevalent. It is believed that the higher temperatures of steel (2750 to 3000° F.) cause the sand additive to bypass the “fluxing” stage and actually melt. The result is a core with no plasticity. In a core with no plasticity, as the thermal expansion of the sand takes place, the surface integrity of the core is broken and veining occurs. The phenomenon also occurs in iron castings where the temperature at the sand core/molten iron interface is greater than 2750° F. These regions, known as “hot spots,” are where the geometries of the castings and sand cores allow for thick metal sections (greater than three inches) to be in contact with thin sand sections. The heat generated cannot be dissipated, and the sand additive “melts” and is rendered ineffective.
Accordingly, a need exists for an additive which effectively combats the thermal expansion of chemically bonded sand at temperatures higher than those of the pouring temperatures of molten iron. Such an additive must eliminate veining defects in steel castings and in iron castings with “hot spots,” where the existing commercial products such as Veinseal 14000 and others are ineffective.